A variety of automatic vehicle navigational systems has been developed and used to provide information about the actual location of a vehicle as it moves over streets. A common purpose of the vehicle navigational systems is to maintain automatically knowledge of the actual location of the vehicle at all times as it traverses the streets (i.e., track the vehicle). A given navigational system may be utilized in the vehicle to provide the vehicle operator with knowledge of the location of the vehicle and/or at a central monitoring station that may monitor the location of one or more vehicles.
For example, one general approach to such vehicle navigational systems is known as "dead reckoning", in which the vehicle is tracked by advancing a "dead reckoned position" from measured distances and courses or headings. A system based upon dead reckoning principles may, for example, detect the distance traveled and heading of the vehicle using distance and heading sensors on the vehicle. These distance and heading data are then processed by, for example, a computer using known equations to calculate periodically a dead reckoned position DRP of the vehicle. As the vehicle moves along a street, an old dead reckoned position DRP.sub.o is advanced to a new or current dead reckoned position DRP.sub.c in response to the distance and heading data being provided by the sensors.
One problem with prior systems using dead reckoning is the accumulation of error that occurs as the dead reckoned positions are advanced. This error occurs, in part, as a result of inherent limitations on the achievable accuracy of the distance and heading sensors, which thus provide data that do not precisely identify the distance traveled nor the heading of the vehicle. Unless compensation for this error is made, the dead reckoned positions will become increasingly imprecise or inaccurate.
Prior dead reckoning vehicle navigational systems have been developed and have attempted to solve this problem of the accumulation of error by providing additional information to the dead reckoned positions. Generally, the additional information may be a map corresponding to the streets of a given area over which the vehicle may be moving. The map is stored in memory as a map data base and is accessed by the computer to process this stored information in relation to the dead reckoned positions.
U.S. Pat. No. 3,789,198, issued Jan. 29, 1974, discloses a vehicle location monitoring system using dead reckoning for tracking motor vehicles, including a technique for compensating for accumulated errors in the dead reckoned positions. In this system, a computer accesses a stored map data base, which is a table or array having a 2-dimensional, orthogonal grid of entries of coordinates X.sub.st Y.sub.st that may or may not correspond to driveable surfaces, such as streets St. Storage locations in the array that correspond to streets are indicated by a logic 1, while all other storage locations are filled with a logic 0.
In accordance with a vehicle navigational algorithm of the patent, a dead reckoned position DRP of the vehicle is periodically calculated, which position DRP is identified and temporarily stored in the computer as coordinates X.sub.old Y.sub.old. Then, to compensate for the accumulated error, the array is interrogated at a location corresponding to the coordinates X.sub.old Y.sub.old. If a logic 1 is found, the vehicle is defined as corresponding to a known driveable surface and no correction is made. If a logic 0 is found, representing no driveable surface, adjacent entries in the array are interrogated, as specifically described in the patent. If a logic 1 is then found at one of these adjacent entries, coordinates X.sub.old Y.sub.old are corrected or updated to coordinates X.sub.st Y.sub.st corresponding to the logic 1 that was found, and these latter coordinates then become X.sub.old Y.sub.old to advance the dead reckoned position. If no logic 1 is found after such interrogations, then no change is made to the original X.sub.old Y.sub. old and the corresponding dead reckoned position DRP is advanced.
Another example of an automatic vehicle navigational system that uses a map data base to correct for the accumulation of errors in tracking a vehicle is disclosed in a publication entitled "Landfall: A High Resolution Vehicle-Location System", by D. King, GEC Journal of Science and Technology, Vol. 45, No. 1, 1978, pages 34-44. As described in the publication, the term Landfall is an acronym for Links and Nodes Database For Automatic Landvehicle Location, in which a stored map data base comprises roads (links) that are interconnected by junctions (nodes) having inlet/outlet ports. Thus, any mapped area is regarded merely as a network of nodes, each containing a number of inlet/outlet ports, and interconnected links.
The publication describes the basic vehicle navigational algorithm used under the Landfall principle by assuming that a vehicle is on a road or link moving toward a node which it will enter by an input port. As the vehicle moves forward, the motion is detected by a distance encoder and the "distance-to-go", i.e., the distance to go to the next node, is decremented until it becomes zero, corresponding to the entry point of the input port of such a node. Then, as the vehicle exits one of several output ports of the node, a change of heading of the vehicle at the exit point with respect to the entry point is measured. Then, the map data base for that node is scanned for an exit port matching the measured change in heading and, once identified, this exit port leads to the entry point of another node and the distance-to-go to that other node. Landfall attempts to compensate for the accumulation of error resulting from the achievable accuracy of the distance encoder by cancelling the error when the vehicle encounters a node and turns onto an exit port. More details of this vehicle navigational algorithm are disclosed in the publication.
A common problem with the above-mentioned systems is the use of limited information to compensate for the accumulation of error, so as to accurately track a vehicle. For example, in the vehicle navigational system of the patent, this limited information is a coarse and simplistic representation of streets by logic 1 and logic 0 data of the map data base. In the Landfall system, a relatively simplistic assumption is made that vehicles are always on a street of the map.
Furthermore, in addition to using limited information to correct for the accumulation of error, the vehicle navigational algorithms of the patent and Landfall do not develop an estimate of correct location accuracy and use this information in dependence with the map data base to determine if the vehicle is on a street or not. Systems that do not maintain this estimate are more likely to update the position incorrectly or to fail to update the position when it should be.